35' cat concept for the inside passage.

which is why I threw ( or one reason why ) in the rocker for and aft, I'd have been content with an arc side to side but it seemed like a shorter arc might be to much stress on the ply, so to accommodate some of the suggestions and to soften the bend I went for and aft. I also thought that since I was loosing the flat line for and aft I wouldn't very easily get a compound bend. So again if I loose one aspect of the Bolger form I loose several.

Oh well most folks seemed to like the new hull form and it does give me quite a bit more elbow room in the hulls

 

Negligible? i dont think so... The idea is to carry as much displacement as possible using the flat bottomed U shape hull as it has the least wetted area per length and waterline beam. The round bilge or chines were only to fair in the bows, provide a fine entry and to prevent harsh slamming in a seaway... If you were to use a full multichined hull throughout the midbody and stern, this would have the most wetted area and be the least desirable from a resistance point of view. True round would be better, flat bottomed is best...

 

well I had a flat bottomed u shaped hull on there and it seemed pretty unpopular, Its easier in terms of building and it has less connections so its lighter as well.

Oh and the coast guard approval thing. I'd not build anything without it first being stamped by an NA and approved, but I think for a six pack the regs aren't that bad, railing height, cat walk maybe, things like that.

 

Mass 40x21 is more boat than what I want, besides I checked the parking situation up in Ketchican and they are tight on moorings over 38' so keeping it under that is kinda key if I want to easily find a place to put this thing. I'm not particularly interested in rafting a thin skinned cat between a couple steel barges at some crowded dock. So I'm trying to make some accommodations for the home port. Hook will work fine by me but I gotta keep within the swing/length limit.

I did kinda like the idea of two different sized engines, Interesting concept. Might have to look into that one.

 

Us monohull guys always work with One high quality engine . one bow thruster and one high quality generator. Can a cat operate the same ?

 

can't imagine why not, but there is something to be said for safety in numbers. I was leaning towards one larger engine rather than two smaller ones but I could use two and slave them in to the same drive system so only one need be running if I wanted a slow cruise mode. I wasn't really thinking I'd need more than something in the displacement range of speed anyway though

 

Well if you had a 10 or 15 hp outboard for your tender that could easily be hung on a hull , it would supply useful emergecy power on a coastal cruiser.

The thought of two main engine instalations brings dollar signs to my eyes. What a waste of money ,space , not to mention the weight.

 

hadn't explored the idea yet but I'll be looking into an electric launch eventually. I could use the bats in the launch as the house bank and charge off the main engine or gen set. That way I wouldn't have two different fuel types. Diesel outboards are damn hard to find. I could use have say two batteries on board and two in the launch, slave them together. Could work out specially if I'm only going from the hook to shore the vast majority of the time.

 

Ive not yet seen a practical electric tender. Tenders need to be able to be man handled by one person onto a beach .... And they need to be able to survive a swamping.


Speaking of tenders and engines...today is a very good time to purchase a two stroke. They are becoming extremely rare new. The 9.9 hp Two stroke is the tender power king. 25 percent lighter than a 9.9hp 4 stroke . Fantastic motor

Check these guys out with your credit card before its to late.

http://www.boats.net/outboard_motor/Yamaha/9.9HP/Y-9.9MSH.html

 

Hi Groper,
Change in resistance is minimal compared to wave making and so forth... The cost of a ROUNDED section in the hull is not justified by the cost unless you are in a VERY tight racing competition... and clean your 'wetside' every week - do consider reality... In theory yes all very nice but the cost benefit for a cruising boat is not justified on performance difference... 3 chine form is a nice and easy compromise which is quick to build and offers an efficient performance consideration...

Hi Boston,
Electric drive is not sufficiently mature, (I seriously considered that option and still have the generator - 20hp kubota with a pair of DELCO alternators to give 6000 watts continuous at 29 or 58 volts DC), and batteries/power cabling are a major weight penalty & cost factor and the diesel is heavier with the generator part... My pair of 21hp Kubota sail-drives come in at 140kg each (engine, gearbox, controls, drive leg and propeller), the "OEM kit", includes wet exhaust, fittings, fuel lines and mounting assembly - everything needed except the epoxy and optional braces inside the mounting assembly - made of moulded grp... Each engine will push my boat at better than 6 knots at cruise burning 3 litres/hour... - It is your boat and there is lots of good and well intentioned advice presented by all the contributors... Your difficult task is to pick the eyes out of what you want and mould those ideas and opinions into something that will hopefully suit your needs and intentions...

 

yah I haven't put a lot of thought into the tender yet.

 

Masalai, i know exactly what you are talking about and for the most part i agree with you.

But if your going to design a hull, why not do it as best as possible with the minimum resistance possible within a set of design constraints? Designing the entire boat is compromise after compromise, so why add unnecessary compromises when you are already faced with so many?

What i see in modern multihull designs, is quite different to what was done years ago. I think a large part of the reason for this is advancements in fluid dynamics analysis. The sleek shapes you see in americas cup racing yachts has a function, its not just for fancy looks. The best (lowest resistance) catamaran hulls ive modelled to date look basically identical to the americas cup cats and tri hulls even though i have been using much more moderate length/beam ratios than the racing yachts do... ive concluded from all teh hydrostatic data, that the shape works so well because it minimises both viscous drag and wave making drag for a given length and beam. The shape achieves this my minimizing wetted area for a given length/beam ratio and displacement, and minimizes wave drag by generating a longer wavelength bow wave train into which the midbody serves as a sink, resulting in overall less wave resistance.

20 years ago, this information was very difficult to come by... and boat builders are a stubborn bunch reluctant to change especially when youve invested big money in molds etc... so its taken quite some time longer than it should for this evolution to happen.

Slowly but surely, i see the same basic underwater hull forms in all of the best modern multihulls;
Compare these, they all share the common theme im talking about;





The list goes on, but notice the repetition? They all basically the same shape, just with different length/beam ratios and displacements...

 

And they are ALL bloody HUGE MONSTERS, built by highly skilled tradesmen who have a regular salary to do that work - Horses for courses.... and I do not have multiple millions to waste on "dockside presentation with pretty coloured lights below the water line and MONSTROUS escape hatches ? - - Or the billions to maintain 110% performance standard as an out & out racing machine...

 

it doesn't look like a particularly hard shape to build either. even in ply. I'd just form the tighter radii with strip plank and then transition into the ply for the more mild areas. Seems like it would work well enough.

They are big Mass but the principals are still the same mate. Costs the same to feed and vet a good horse as it does a bad one.

 

To continue abstractly with horses, - - - but harnessing an 'Arab thoroughbred' to a plough is plain silly... or entering a draught horse (Clydesdale) in the 'Kentucky Derby', - laughable...